This invention relates to phosphorus/vanadium oxide catalysts useful in a process for the oxidation of hydrocarbons to dicarboxylic acid anhydrides, and more particularly to a high surface area catalyst of improved microstructure which provides high yields in such a process. The invention is also directed to a method for the preparation of the catalyst.
Numerous catalysts containing vanadium, phosphorus and oxygen (sometimes referred to as mixed oxides of vanadium and phosphorus), substantially in the form of vanadyl pyrophosphate, optionally containing a promoter component, are disclosed in the prior art as being useful for the conversion of various hydrocarbon feed stocks to maleic anhydride. In general, such catalysts wherein the valence of the vanadium is less than +5, usually between about +3.8 and about +4.8, are considered particularly well suited for the production of maleic anhydride from hydrocarbons having at least four carbon atoms in a straight chain (or cyclic structure). Typically, such catalysts also contain promoter elements or components which are considered to be present in the catalyst as oxides. Common organic feed stocks include non-aromatic hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof.
Generally, such catalysts are prepared by contacting vanadium-containing compounds, phosphorus-containing compounds, and promoter component-containing compounds (when a promoter element is desired) under conditions sufficient to reduce pentavalent vanadium to the tetravalent state and form the desired catalyst precursor comprising vanadyl hydrogen phosphate, optionally containing a promoter component. The catalyst precursor is thereafter recovered, typically in particulate form, and subjected to a variety of further conventional processing techniques to produce the active catalyst. An essential step in such further processing is calcination. Prior to calcination, the catalyst is typically formed into a shaped body such as tablet or pellet by compression in a die. A lubricant is ordinarily incorporated in the precursor composition to facilitate the tableting or pelletizing process.
In its final form, the catalyst comprises a mass of porous tablets or pellets which are charged in bulk to provide the catalyst bed of a fixed bed reactor. Typically, the catalyst is charged to a tubular reactor comprising the tubes of a shell and tube heat exchanger. Hydrocarbon and oxygen are fed to the tubes, and a heat transfer fluid, such as molten salt, is circulated through the shell to remove the exothermic heat of the oxidation reaction. The porous nature of the catalyst contributes substantially to the active surface area at which the catalytic reaction takes place. However, for the internal surfaces of the catalyst body (tablets or pellets) to be utilized effectively, the feed gases, hydrocarbon and oxygen, must diffuse through the pores to reach the internal surfaces, and the reaction products must diffuse away from those surfaces and out of the catalyst body.
It is known in the art that resistance to internal diffusion in the catalyst bodies can become a rate limiting factor in the reaction. The diffusion paths can be shortened (and catalyst body external surface increased) by using relatively small catalyst granules. However, in this case better mass transfer is purchased at a sacrifice in pressure drop through the fixed bed. Thus, a need has existed in the art for a phosphorus/vanadium oxide catalyst having a microstructure such that internal diffusion resistance is minimized and productivity is enhanced at constant granule size and pressure drop.
Zazhigalov, et al, "Effect of the Pore Structure and Granule Shape of V-P-O Catalyst on the Selectivity of Oxidation of n-Butane," Zhurnal Prikladnoi Kimii, Vol. 61, No. 1, pp. 101-105 (January, 1988) reports that the activity of V-P-O catalysts in the oxidation of n-butane increases with an increase in the total pore volume and macropore volume. Zazhigalov et al further describe the use of polyethylene oxide as a pore forming additive in the preparation of V-P-O catalyst to produce granules having a greater proportion of macropores. The pore builder is apparently incorporated in the catalyst precursor formulation, and later removed from the catalyst by burning it out in the calcination step. This process produces a catalyst having a proportion of macropores significantly greater than was realized without the pore builder. However, despite the previously recognized advantage of macropores, test reactor experiments show that the addition of polyethylene oxide resulted in a decrease in the efficiency of the catalyst. Zazhigalov et al. explains these results by assuming that, after burn-up of the polymer, a dense film of coke remains on the surface of the catalyst and deactivates the active centers of the catalyst. They confirmed that hypothesis by the detection of CO.sub.2 that was liberated when the catalyst was heated (873.degree. K.) in an air current.
Mount et al. U.S. Pat. No. 4,092,269 is directed to a phosphorus/vanadium oxygen catalyst prepared by reaction of phosphoric acid, phosphorus acid and vanadium pentoxide in an aqueous medium to produce a catalyst precursor, which is converted to an active catalyst by calcination. The catalyst produced contains predominantly pentavalent vanadium, generally having an average vanadium oxidation state of about +4.6 or more. The B.E.T. surface area of the Mount catalyst is about 8 m.sup.2 /g or less, and the pore volume of the catalyst from pores having diameters between about 0.8 microns and about 10 microns is greater than 0.02 cc/g. Preferably, the volume constituted of pores having diameters between 1 and 5 microns is at least about 0.03 cc/g. However, Mount states that catalysts having a pore volume from pores having diameters larger than about 10 microns have virtually no effect on the yield of maleic anhydride using such catalysts. Catalysts having Mount's desired fraction of 0.8 to 10 micron macropores are prepared by adding a pore modification agent to the precursor at any stage prior to calcination. Calcination of the precursor containing the pore modification agent is conducted at a temperature between about 300.degree. and 600.degree. C. A lengthy list of pore modification agents is disclosed, including adipic acid, citric acid, oxalic acid, stearic acid, polyethylene glycol, polyvinyl alcohol, polyacrylic acid, cellulosic materials, monosaccharides, polysaccharides, hydrogenated vegetable oils, waxes, and gelatin. Cellulosic materials and hydrogenated vegetable oils are preferred, and methylcellulose especially preferred. The Mount et al. reference states that the yield of maleic anhydride using a phosphorus/vanadium oxide catalyst is significantly improved by controlling the pore size distribution of the finished catalyst in the ranges discussed above. In the working examples of Mount et al., the pore modification agent is removed by calcining at 380.degree. to 500.degree. C.
Bither U.S. Pat. No. 4,699,985 describes the preparation of a maleic anhydride catalyst in which a precursor catalyst is blended with 3 to 5% by weight of an organic pore modifying agent, and with fumed silica in an amount of 0.05 to 0.20% by weight. Upon firing of the blend, the organic pore modifying agent and the fumed silica generate a catalyst microstructure which is said to lead to enhanced production of maleic anhydride. Pore builders disclosed as suitable include organic acids, polymeric materials, cellulosic materials, monosaccharides and polysaccharides, hydrogenated vegetable oils and waxes. A preferred pore builder is Sterotex hydrogenated cottonseed oil. The pore modifying agent also serves as a lubricant in preparing shaped catalyst particles. According to the disclosure, the precursor blend is fired in a controlled manner to generate and activate the catalyst species. Precursor catalyst pellets are heated in a low flow of air at 375.degree. to 400.degree. C. for 1 to 6 hours, and thereafter in a more rapid flow of 1 to 1.5% n-butane in air at 450 .degree. to 490.degree. C. for an additional 16-24 hours. In a preferred method, the shaped catalyst precursor blend is initially fired in a continuous zoned belt furnace in an air atmosphere. The temperature varies from ambient at the furnace ends to 390.degree. to 395.degree. C. at the center of the heated zone. Air diffuses through baffles at the ends of the furnace to replace combustion products diffusing out through vertical vents located in the heated zone of the furnace.
Methods have been developed in the art for the preparation of high surface area catalysts by reaction of vanadium pentoxide and a phosphorus compound in an organic medium. The surface area of these catalysts is generally in the range of 15 m.sup.2 /g or greater as determined by the method of Brunauer, Emmett and Teller, J. Am. Chem. Soc., 60, 309 (1938). Surface area as determined by this method is generally referred to in the art as "B.E.T." surface area. Methods for producing high surface area catalysts are described, for example, in U.S. Pat. Nos. 4,632,916; 4,632,915; 4,567,158; 4,333,853; 4,315,864; 4,328,162; 4,251,390; 4,187,235; and 3,864,280.
Copending and coassigned application Ser. No. 07/722,070, filed Jun. 27, 1991, now U.S. Pat. No. 5,137,860 describes a process for the transformation of a catalyst precursor represented by the formula: EQU VO(M).sub.m HPO.sub.4 .multidot.aH.sub.2 O.multidot.b(P.sub.2/c O).multidot.n(organics)
into an active catalyst represented by the formula: EQU (VO).sub.2 (M).sub.m P.sub.2 O.sub.7 .multidot.b(P.sub.2/c O)
where M is at least one promoter element selected from the group consisting of elements from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, and VIIIA of the periodic table, and mixtures thereof, m is a number from zero (0) to about 0.2, a is a number of at least about 0.5, b is a number taken to provide a P/V atom ratio of from about 1.0 to about 1.3, c is a number representing the oxidation number of phosphorus and has a value of 5, and n is a number taken to represent the weight % of intercalated organic components in the precursor. In transforming the precursor to the active catalyst, the precursor is heated in an atmosphere of air, steam, inert gas, and mixtures thereof to a temperature not greater than about 300.degree. C. The catalyst precursor is maintained at such temperature under an atmosphere containing molecular oxygen, steam, and optionally an inert gas, the atmosphere being represented by the formula (O).sub.x (H.sub.2 O).sub.y (IG).sub.z where IG is an inert gas and x, y, and z represent mol percentages of the O.sub.2, H.sub.2 O, and IG components, respectively, in the molecular oxygen/steam-containing atmosphere, x having a value greater than zero (0) mol percent, y having a value greater than zero (0) mol % but less than 100 mol %, and z having a value representing the balance of the molecular oxygen/steam-containing atmosphere. The temperature is increased at a programmed rate of from about 2.degree. C./min. to about 12.degree. C./min. to a value effective to eliminate the water of hydration from the catalyst precursor. The temperature is then adjusted to a value greater than 350.degree. C., but less than 550.degree. C. and the adjusted temperature is maintained in the molecular oxygen/steam-containing atmosphere for a time effective to provide a vanadium oxidation state of from about +4.0 to about +4.5. The adjusted temperature is maintained thereafter in a non-oxidizing, steam-containing atmosphere for a time effective to complete the catalyst precursor-to-active catalyst transformation to yield the active catalyst.